Sintered body of silicon nitride and method of producing the same

ABSTRACT

The invention reduces the time required for nitriding in the process of reaction sintering for production of a sintered body of silicon nitride, thereby improving productivity, and provides a sintered body of silicon nitride having sufficient compactness and high strength which can be produced by reaction sintering. The sintered body is Si 3  N 4  having an unpaired electron density of 10 15  /cm 3  to 10 21  /cm 3 . The sintered body is produced through reaction sintering by using a Si powder having an unpaired electron density of 10 15  -10 20  /cm 3 , which is obtained by annealing a commercially available Si powder at temperatures of 300° to 800° C. in other than nitrogen atmosphere for 3-5 hours. In particular, the sintered body is produced by adding to the so obtained Si powder, together with a sintering assistant, an element having a valence of, particularly, from +1 to +3, the element having a covalent bond radius RM which bears such a relation with the covalent bond radius RSi of Si that (RM - RSi)/RSi&lt;0.5, or a compound of the element, as a nitrogen hole producing agent, and subjecting the resulting composite to reaction sintering.

TECHNICAL FIELD

The present invention relates to a sintered body of silicon nitride (Si₃ N₄) and a method of producing the same.

BACKGROUND ART

Reaction sintering has been known as one of the methods for production of a sintered body of silicon nitride wherein a shaped body of silicon (Si) powder is nitrified and sintered at the same time in a nitrogen atmosphere. However, in order to obtain a sintered body of silicon nitride by causing a shaped body composed of pure Si powder to react with nitrogen, it is necessary that the shaped body be subjected to a prolonged process of nitriding which extends for more than 100 hours as stated in, for example, J. Mater. Sci. 22 (1987), pp. 3041-3086, more particularly, pp. 3074-3075. In this way, reaction sintering has involved a large drawback of low productivity.

Methods for accelerating the process of nitriding through addition of a nitriding promotor, such as Ni, Co, Ti, or Zr, to Si powder have been proposed in Japanese Patent Publication No. 61-38149, Japanese Patent Application Laid-Open No. 5-330921, and Japanese Patent Application Publication No. 5-508612. According to these methods, an amorphous SiO₂ layer on the surface of the Si powder reacts with the nitriding promotor to turn into liquid phase so that the rate of nitrogen diffusion is increased. Thus, it is considered that the process of nitriding is accelerated accordingly. According to JPA Laid-Open No. 5-330921 in particular, time required for nitriding can be reduced to about 8 hours.

However, in these methods which use a nitriding promotor, the coefficient of nitrogen diffusion within the Si₃ N₄ layer formed on the inner side of the liquid phase on the surface of Si powder is insignificant so that any further inward nitrogen diffusion and any further Si₃ N₄ creation are prevented. Therefore, any further reduction in the time required for nitriding could not be expected. Further, as described in JPA Laid-Open No. 5-330921, complex procedures of temperature control are required such that nitriding treatment is carried out in several steps at intervals of 50° C. under high temperature conditions on the order of 1200° to 1450° C. As such, it can hardly be said that such method has good productivity advantage.

Further, sintered bodies of Si₃ N₄ as produced under the reaction sintering techniques using such nitriding promoters have been found unsuitable for use as structural material, because they are not sufficiently compact in texture having a porosity of 4 to 11 vol. %, and because their strength characteristic is rated poor with a three-point bending strength of 460 MPa.

In view of these problems of the prior art, it is an object of the present invention to reduce the time required for the process of nitriding in reaction sintering for production of a sintered body of silicon nitride, thus improving productivity, and to provide a sintered body of silicon nitride having sufficient compactness and high strength which can be produced by reaction sintering.

DISCLOSURE OF THE INVENTION

In order to accomplish the foregoing object, the present invention provides a sintered body of silicon nitride produced by reaction sintering of Si powder, characterized in that its unpaired electron density is 10¹⁵ /cm³ -10²¹ /cm³.

To produce the sintered body of silicon nitride of the present invention, reaction sintering is carried out using a Si powder having an unpaired electron density of 10¹⁵ /cm³ -10²⁰ /cm³. Si powder having unpaired electrons within such a density range can be obtained by annealing any commercially available Si powder at temperatures of 300°-800° C. in other than nitrogen atmosphere for 1 to 5 hours. The atmosphere for this process is preferably air, hydrogen, argon, their mixtures, or a vacuum of not more than 10 torr.

For the sintered body of silicon nitride according to the invention, a sintering assistant and a nitrogen vacancy producing agent powder are added to a Si powder having an unpaired electron density as mentioned above, and their composite is subjected to reaction sintering.

More specifically, in the method of producing a sintered body of silicon nitride according to the invention, a Si powder having an unpaired electron density within the above mentioned range is loaded with a compound of at least one kind of element selected from among rare earth element, Al, Mg, and Ca, as a sintering assistant in an amount of 0.1 to 15 mol %, preferably 0.5 to 10 mol %, in terms of element value, and an element other than a rare earth element, Al, Mg, and Ca having a valence of from +1 to +4, preferably +1 to +3, the element having a covalent bond radius RM which is in such a relationship with the covalent bond radius RSi of Si as (RM-RSi)/RSi<0.5, or a compound of the element, as a nitrogen vacancy producing agent in an amount of 0.5 to 15 mol %, preferably 1 to 10 mol %, in terms of element value, and the resulting composite is subjected to reaction sintering.

A specific example of nitrogen vacancy producing agent is at least one kind of element selected from among Sc, Ni, V, Cr, Mn, Fe, Co, Cu, Ti, Zn, Ga, and Ge, or a compound thereof, preferably alkoxide, stearate, or laurate of any such element.

In conventional reaction sintering of silicon nitride, the presence of two impediments against nitrogen diffusion, namely, an amorphous SiO₂ layer formed on the surface of Si powder and a Si₃ N₄ layer formed on Si powder as a result of nitriding, is responsible for such long time process of nitrogen treatment being required as already mentioned.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a commercially available Si powder;

FIG. 2 is a schematic view of a Si powder having its Si(O) layer increased in thickness by heat treatment;

FIG. 3 is a schematic view of a Si powder having a SiM layer such that an impurity element M is dissolved as solid in a Si(O) layer;

FIG. 4 is a schematic view of a Si powder having a SiMV layer such that impurity element M is diffused into a Si layer;

FIG. 5 is a graph showing the relationship between treating temperature for commercially available Si powder having an unpaired electron density of 7×10¹² /cm³ and the unpaired electron density of the Si powder obtained; and

FIG. 6 is a graph showing the relationship between treating temperature for a Si powder having an unpaired electron density of 2×10¹³ /cm³ and the unpaired electron density of the Si powder obtained.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, two characteristic steps are adopted in order to overcome the aforesaid two impediments against nitrogen diffusion, whereby considerable improvements have been successfully achieved such that the rate of nitrogen diffusion is increased and the time required for nitriding is substantially reduced. The two characteristic steps are the step of introducing silicon vacancies into Si powder, a material powder to be used, and the step of creating nitrogen vacancies within Si₃ N₄.

A commercially available Si powder is believed to be of three-layer construction as shown in FIG. 1. That is, the powder consists of a surface layer or amorphous SiO₂ layer 1₀ formed as a result of spontaneous oxidation of Si, a Si(O) layer 2₀ located inward thereof and having an oxygen solid solution phase formed in Si, and an innermost Si layer 3. The surface amorphous SiO₂ layer 1₀ inhibits inward diffusion of nitrogen as earlier stated. In the middle layer or Si(O) layer 2₀, solid solution oxygen is present between lattices, and inter-lattice oxygen increases lattice constant of Si.

According to the invention, therefore, in order to inhibit any increase in the lattice constant of Si(O) layer 2₀, silicon vacancies are created. Specifically, commercial silicon powder is subjected to proper heat treatment or the like and, as FIG. 2 shows, the oxygen in the surface amorphous SiO₂ layer 1₁ is caused to diffuse inward thereby to reduce the thickness of the amorphous Sio₂ layer 1₁ and increase the thickness of the Si(O) layer 2₁ so that a large number of silicon vacancies are present therein. It is believed that such silicon vacancies are operative to increase the rate of diffusion of elements dissolved as solid in Si.

Next, nitrogen vacancies will be explained which are created within Si₃ N₄ lattice in order to increase the speed of nitrogen diffusion within Si₃ N₄ created by nitriding. The Si₃ N₄ lattice is so configured as to permit +4 valent Si and -3 valent N to be kept electrically neutral. When elements of, for example, +1 to +3 valence are dissolved as solid within the Si₃ N₄ lattice, an element having plus charge will occupy lattice points of Si and create nitrogen vacancies in order to retain its electric neutrality.

For example, when M ions of +3 valence become dissolved as solid by X in a Si₃ N₄ crystal, the formation of nitrogen vacancies may be expressed by the following reaction formula:

    Si.sub.3 N.sub.4 +xM→Si.sub.(3-x) N.sub.(4-x/3) M.sub.x VN.sub.(x/3)(1)

where VN represents a nitrogen vacancy.

Any Si₃ N₄ layer having a large number of such nitrogen vacancies will have an increased supply of nitrogen into the Si layer located inward of the Si₃ N₄ layer, because nitrogen diffusion via the vacancies is accelerated. Thus, it is believed that the speed of nitriding can be dramatically enhanced. In case that ions of +4 valence or higher are dissolved as solid, the quantity of plus charge is excessive; therefore silicon vacancies, rather than nitrogen vacancies, are created and any drastic increase in nitriding speed is no longer expectable. However, even when M ions of 4 valence, which is the same with Si⁴⁺, are added, only using a nitrogen vacancy producing agent having an RM larger than RSi and giving an unpaired electron density range of 10¹⁵ /cm³ to 10²⁰ /cm³ to the Si powder, for example, will produce an effect of suppressing any increase in the lattice constant of Si, as will be described later. In this case, although the result would not be comparable to that of elements of +1 to +3 valence, yet the nitrogen diffusion is accelerated so that the time required for nitriding can be shortened.

In the present invention, it is desirable to use elements of, particularly, +1 to +3 valence. In this case, aforesaid Si(O) layer 2 is utilized to dissolve those elements of +1 to +3 valence as solid in the Si₃ N₄ which has been formed through nitriding. That is, the element is dissolved as solid in silicon vacancies which exist in multiplicity in the Si(O) layer 2. Where quantity z of such element M is dissolved as solid in silicon vacancies, the following reaction formula holds:

    Si(O).sub.(1-y) Vsi.sub.(y) +zM→Si(O).sub.(1-y) M.sub.z Vsi.sub.(y-z)(2)

where Vsi represents a silicon vacancy.

In this invention, therefore, Si powder in such condition consists mainly of Si(O).sub.(1-y) M₂ Vsi.sub.(y-z) layer (hereinafter referred to as SiM layer) 2₂, and an interior Si layer 3. As diffusion progresses further, quantity u of M is diffused from SiM layer 2₂ into Si layer 3, whereby Si.sub.(1-u) M_(u) layer is formed. In this case, if the covalent bond radius of element M is larger than that of Si, as FIG. 4 shows, creation of silicon vacancies is accelerated to inhibit the lattice constant of Si from becoming larger and thus Si.sub.(1-u-w) M_(u) Vsi_(w) layer (hereinafter referred to as SiMV layer) 4 is created.

The nitriding treatment causes SiM layer 2₂ and SiMV layer 4 of the powder to react respectively in manner as the following reaction formulas indicate:

    3Si.sub.(1-y) (O)M.sub.z Vsi.sub.(y-z) +2N.sub.2 →Si.sub.3(1-y) (O)N.sub.(4-3y+9z/4) M.sub.3z VSi.sub.3(y-z) VN.sub.(4y-3z)(3)

    3Si.sub.(1-u-w) M.sub.u Vsi.sub.w +2N.sub.2 →Si.sub.3(1-u-w) N.sub.(4-u-4w) M.sub.3u Vsi.sub.3w VN.sub.(u+4w)          (4)

In the formulas, VN represents a nitrogen vacancy; and M is of +3 valence.

As may be appreciated from the foregoing reaction formulas, nitrogen vacancies are created in either Si₃ N₄ layer, it being thus expectable that the rate of nitrogen diffusion will grow larger within the Si₃ N₄ layer.

As may be understood from the foregoing description, with the prior art methods such that a nitriding promotor is merely added to Si powder, as described in Japanese Patent Publication No. 61-38149, Japanese Patent Application Laid-Open No. 5-330921, and Japanese Patent Application Publication No. 5-508612, the Si(O) layer is so thin that the quantity of nitriding promotor solid solution present therein is insufficient to enable formation of SiM and SiMV layers within the Si powder. Therefore, no nitrogen vacancy is formed within the Si₃ N₄ layer produced through the process of nitriding. This results in insignificant rate of nitrogen diffusion, which in turn results in a prolonged process of nitriding.

Next, the process of producing a sintered body of Si₃ N₄ by reaction sintering in accordance with the invention will be explained in detail. Initially, commercial Si powder is annealed at temperatures of 300°-800° C. for 1 to 5 hours. By so doing the oxygen of the amorphous SiO₂ layer formed on the powder surface is diffused into Si to form a Si(O) layer in which Si vacancies are present in multiplicity. Provided that the treating atmosphere must be an atmosphere other than nitrogen atmosphere, that is, air, hydrogen or argon, or a vacuum of not more than 10 torr. Treatment in a nitrogen atmosphere results in the formation of a Si₃ N₄ film on the surface of Si powder, which makes it impracticable to allow the nitrogen vacancy producing agent to be dissolved as solid.

For the formation of a nitrogen vacancy Si(O) layer, it is not only possible to employ the aforesaid annealing technique, but also to use another technique, such as a method for implantation of oxygen ions into Si powder or a method for forcibly including oxygen during bulk Si preparation.

The number of Si vacancies in the Si(O) layer in the Si powder thus obtained can be quantitatively determined as the number of unpaired electrons trapped in Si vacancies within the Si(O) layer by using the electrospin resonance technique (ESR technique). As a result of such measurement, it was found that when the unpaired electron density of Si powder was controlled within the range of 10¹⁵ -10²⁰ /cm³ by aforesaid technique, nitriding reaction in particular could be highly accelerated, as against the unpaired electron density of commercial Si powder which was 10¹² -10¹³ /cm³.

If the number of unpaired electrons, that is, the number of Si vacancies is smaller than 10¹⁵ /cm³, the Si vacancies are insufficient to accelerate the formation of solid solution of the nitrogen vacancy producing agent. If the number of unpaired electrons is larger than 10²⁰ /cm³, the process of nitriding may be accelerated, but the quantity of oxygen in Si crystals is excessively large, so that residual oxygen in Si₃ N₄ crystals or vacancies therein may cause a decrease in strength, the resulting sintered body of Si₃ N₄ being nothing but one of such low strength as not more than 800 MPa in three-point bending strength.

Next, the Si powder of high unpaired electron density as obtained in above described manner is mixed with a sintering assistant comprising a compound composed of at least one kind of element selected from among rare earth element, Al, Mg and Ca, and a nitrogen vacancy producing agent, and the mixture is molded into shape. The nitrogen vacancy producing agent is an element other than a rare earth element, Al, Mg, and Ca having a valence of from +1 to +4, desirably from +1 to +3, the element having a covalent bond radius RM which is in such a relationship with the covalent bond radius RSi of Si as (RM-RSi)/RSi<0.5, or a compound of the element.

Elements within above mentioned valence range are likely to become dissolved as solid in Si crystals and are liable to produce nitrogen vacancies. Specifically, Sc, Ni, V, Cr, Mn, Fe, Co, Cu, Ti, Zn, Ga, and Ge may be mentioned as examples of such elements. A nitrogen vacancy producing agent and a sintering assistant may be added in the form of powder, but for purposes of producing an extra large sintered body of Si₃ N₄, they are preferably added in the form of alkoxide, stearate, or laurate so that they may be uniformly dispersed over the surface of the Si powder.

Preferably, the proportion of the sintering assistant is within the range of from 0.1 to 15 mol % in terms of element value, and the nitrogen vacancy producing agent is within the range of from 0.5 to 15 mol % in terms of element value. If the proportion of the sintering assistant is less than 0.1 mol %, or if the proportion of the nitrogen vacancy producing agent is less than 0.5 mol %, their effect cannot be obtained. If both proportions exceed 15 mol %, such agent may precipitate in a grain boundary to act as a starting point of fracture and this does not permit production of any high strength sintered body having a three-point bending strength of more than 800 MPa.

Finally, the resulting shaped body is nitrided and sintered. The temperature pattern required during the process of nitriding and sintering may be controlled in a very simple manner such that the shaped body is held at a temperature of 1300° to 1400° C. for 2-4 hours and is then held at a temperature of 1500° to 1800° C. for 1-3 hours. By this treatment is it possible to obtain a sintered body of Si₃ N₄ having a relative density of 99% or more. In particular, where the unpaired electron density of the Si₃ N₄ sintered body obtained is within the range of 10¹⁵ -10²¹ /cm³, a high strength feature on the order of 800 MPa in three-point bending strength can be achieved.

EXAMPLE 1

A commercially available Si powder having an unpaired electron density of 7×10¹² /cm³ (A) and a commercially available Si powder having an unpaired electron density of 2×10¹³ /cm³ (B) were each used as starting material, and the relationship between heat treat conditions and unpaired electron density was examined with respect to each material.

Samples of commercial Si powder (A) and samples of commercial Si powder (B) were held in different atmospheres including (a) air, (b) hydrogen atmosphere, (c) argon atmosphere, and (d) in a vacuum of 10 torr, at 100° C. intervals in a temperature range of from 100° C. to 900° C., for 5 hours each in the case of Si powder (A) and for 2 hours each in the case of Si powder (B). Thereafter, their unpaired electron density values were measured according to the ESR technique.

For purposes of comparison, samples of same commercial Si powder (A) and same commercial Si powder (B) were held in a nitrogen atmosphere (e), and in a vacuum of 100 torr (f), at 100° C. intervals in a temperature range of from 100° C. to 900° C., for 5 hours each. Thereafter, their unpaired electron density values were measured according to the ESR technique.

The results are shown in FIG. 5 for commercial Si powder (A) and in FIG. 6 for commercial Si powder (B). As may be understood from the results shown, the unpaired electron density of Si powder can be controlled within the range of 10¹⁵ -10²⁰ /cm³ by heat treating commercial Si powder at 300°-800° C. in other than nitrogen atmosphere, say, in an air, hydrogen or argon atmosphere or in a vacuum of not more than 10 torr.

EXAMPLE 2

Samples of commercial Si powder (A) were subjected to heat treatment under such atmospheres and conditions as shown in Table 1 below. Unpaired electron density values of the so treated Si powder samples are also shown in Table 1.

It is noted that when the Si powders (A) and (B) were batch-processed for less than 1 hour under the same temperatures and atmospheres as No. 2 to No. 5 of Table 1, respectively, the unpaired electron density was below 10¹⁵ /cm³ at some places around the center of batch bulk, though the unpaired electron density fell upon 10¹⁵ /cm³ as a whole of batch for both Si powders (A) and (B). From this result, it can be understood that the Si powders must be treated for at least 1 hour at temperatures of 300°-800° C. in order to achieve reliable control for the unpaired electron density to fall within the range of 10¹⁵ to 10²⁰ /cm³ over the entire batch of commercial Si powders.

                  TABLE 1                                                          ______________________________________                                                  Heat treat conditions                                                                          Unpaired electron density                             Sample   Atmosphere °C. × hr                                                                   (/cm.sup.3)                                       ______________________________________                                         1        Ar         300 × 5                                                                           1 × 10.sup.15                               2        Air        500 × 1                                                                           1 × 10.sup.16                               3        Vacuum     500 × 5                                                                           4 × 10.sup.16                               4        Hydrogen   600 × 4                                                                           8 × 10.sup.19                               5        Ar         800 × 5                                                                           1 × 10.sup.20                               6        Ar         600 × 1                                                                           9 × 10.sup.19                               7*       Untreated       7 × 10.sup.12                                   8*       Ar         200 × 3                                                                           1 × 10.sup.14                               9*       Air        900 × 3                                                                           2 × 10.sup.20                               10*      Nitrogen   600 × 3                                                                           2 × 10.sup.14                               ______________________________________                                          (Note) In the table, samples bearing * relate to Comparative Examples. Th      vacuum for Sample 3 is 10 torr.                                          

Added to each Si powder thus obtained were 3 mol % of Y₂ O₃ powder and 5 mol % of Al₂ O₃ powder as sintering assistants, and further added 2 mol % of Cu powder as a nitrogen vacancy producing agent. A thermoplastic resin binder, 4 mol %, was added to the mixture and mixed together, and the resulting mixture was molded into shape by a dry press. Treatment for binder removal was carried out in a nitrogen stream at 600° C. for 2 hours. It is noted in the above connection that between the covalent bond radius RM of Cu and the covalent bond radius RSi of Si there is a relation: (RM-RSi)/RSi=0.5.

Next, each shaped body was subjected to nitriding treatment at 1350° C. in a nitrogen stream for 2 hours and was then sintered at a temperature of 1700°-1800° C. for 1-3 hours as shown in Table 2. Features of respective Si₃ N₄ sintered bodies, including unpaired electron density, relative density, and three-point bending strength, are shown in Table 2.

                  TABLE 2                                                          ______________________________________                                                     Unpaired                                                                   Sinter    electron  Relative                                                                               3-point bend                                       conditions                                                                               density   density strength                                   Sample  (°C. × hr)                                                                  (/cm.sup.3)                                                                              (%)     (MPa)                                      ______________________________________                                         1       1700 × 3                                                                           2 × 10.sup.15                                                                      99      1390                                       2       1800 × 3                                                                           7 × 10.sup.16                                                                      99      1300                                       3       1800 × 3                                                                           2 × 10.sup.16                                                                      99      1426                                       4       1700 × 3                                                                           9 × 10.sup.19                                                                      99      1230                                       5       1800 × 3                                                                           8 × 10.sup.20                                                                      99      1230                                       6       1700 × 1                                                                           8 × 10.sup.20                                                                      99      1090                                       7*      1700 × 3                                                                           2 × 10.sup.14                                                                      92      560                                        8*      1700 × 3                                                                           7 × 10.sup.14                                                                      91      680                                        9*      1700 × 3                                                                           4 × 10.sup.21                                                                      99      780                                        10*     1700 × 3                                                                           9 × 10.sup.14                                                                      89      430                                        ______________________________________                                          (Note) In the table, samples bearing * mark relate to Comparative              Examples.                                                                

It may be appreciated from the foregoing that by holding commercial Si powder at 300°-800° C. in other than nitrogen atmosphere for 1-5 hours, the unpaired electron density of the Si powder can be controlled within the range of 10¹⁵ -10²⁰ /cm³ in a uniform, concentration-evened state in the processed powder, and that by using the Si powder in combination with a sintering assistant and a nitrogen vacancy producing agent and subjecting the same to reaction sintering, a sintered body of Si₃ N₄ can be obtained which has a relative density of 99% or more and a three-point bending strength of 1000 MPa or more.

EXAMPLE 3

Aforesaid commercial Si powder (A) was heat treated at 500° C. in air for 2 hours, and a Si powder having an unpaired electron density of 8×10¹⁵ /cm³ was obtained. Also, commercial Si powder (B) was heat treated at 500° C. in an argon atmosphere for 4 hours, and a Si powder having an unpaired electron density of 2×10¹⁹ /cm³ was obtained.

Further, for comparative purposes, commercial Si powder (A) was heat treated at 500° C. in air for 3 hours, and a Si powder having an unpaired electron density of 1×10¹⁶ /cm³ was obtained. Also, commercial Si powder (B) was heat treated at 500° C. in an argon atmosphere for 4 hours, and a Si powder having an unpaired electron density of 2×10¹⁹ /cm³ was obtained.

Added to each Si powder thus obtained were 3 mol % of Y₂ O₃ powder and 5 mol % of Al₂ O₃ powder as sintering assistants, and further added such a nitrogen vacancy producing agent in such a proportion as shown in Table 3 below. The type of Si powder used for each experiment is shown in Table 3 below, together with its unpaired electron density, kind of nitrogen vacancy producing agent used (in powder form), the valence thereof, the ratio of its covalence bond radius RM to covalence bond radius of Si, i.e., (RM-RSi)/RSi value, and the quantity of the agent loaded.

                                      TABLE 3                                      __________________________________________________________________________                Unpaired                                                                       electron                                                                       density                                                                              Nitrogen vacancy producing agent                                                                 Loading                                     Sample                                                                              Si powder                                                                            (/cm.sup.3)                                                                          Kind Valence                                                                             (RM - RSi)/RSi                                                                         (mol %)                                     __________________________________________________________________________     11   A     8 × 10.sup.15                                                                  Sc.sub.2 O.sub.3                                                                    +3   0.46    2                                           12   A     8 × 10.sup.15                                                                  NiO  +2   0.12    2                                           13   A     8 × 10.sup.15                                                                  V.sub.2 (OC)                                                                        +3   0.21    2                                           14   A     8 × 10.sup.15                                                                  CrN  +3   0.14    2                                           15   A     8 × 10.sup.15                                                                  MnO  +2   0.14    2                                           16   A     8 × 10.sup.15                                                                  Fe.sub.2 O.sub.3                                                                    +3   0.14    2                                           17   A     8 × 10.sup.15                                                                  CuO  +2   0.12    2                                           18   A     8 × 10.sup.15                                                                  CoO  +2   0.13    2                                           19   B     2 × 10.sup.19                                                                  Ti.sub.4 O.sub.3 N.sub.2                                                            +3   0.15    2                                           20   B     2 × 10.sup.19                                                                  ZnO  +2   0.24    2                                           21   B     2 × 10.sup.19                                                                  ZnO  +2   0.24    2                                           22   B     2 × 10.sup.19                                                                  Ga.sub.2 O                                                                          +1   0.27    2                                           23   B     2 × 10.sup.19                                                                  GeO  +2   0.23    2                                           24   A     1 × 10.sup.16                                                                  Unloaded                                                                            --   --      --                                          25   A     1 × 10.sup.16                                                                  MoC  +4   0.25    5                                           26   A     1 × 10.sup.16                                                                  ZrO.sub.2                                                                           +4   0.44    5                                           27   A     1 × 10.sup.16                                                                  In.sub.2 O                                                                          +1   0.50    5                                           28   B     2 × 10.sup.19                                                                  PbO  +2   0.58    5                                           29   B     2 × 10.sup.19                                                                  PbO  +2   0.58    5                                           __________________________________________________________________________

Each lot of Si powder and the sintering assistant and nitrogen vacancy producing agent added thereto were mixed together and compounded into shape in the same manner as in Example 2. Then, each shaped body thus formed was subjected to nitriding treatment in a nitrogen stream at a temperature of 1300° to 1350° C. for 2-4 hours as shown in Table 4 below. Thereafter, sintering was carried out at a temperature of 1600° to 1800° C. for 1-3 hours as shown in Table 4. Features of respective Si₃ N₄ sintered bodies thus obtained, including unpaired electron density, relative density, and three-point bending strength, are shown in Table 4.

                  TABLE 4                                                          ______________________________________                                                                   Unpaired      3-point                                                Sinter    electron                                                                              Relative                                                                              bend                                         Nitriding conditions                                                                               density                                                                               density                                                                               strength                               Sample                                                                               (°C. × hr)                                                                  (°C. × hr)                                                                  (/cm.sup.3)                                                                           (%)    (MPa)                                  ______________________________________                                         11    1350 × 2                                                                           1800 × 3                                                                           7 × 10.sup.16                                                                   99     1340                                   12    1350 × 2                                                                           1800 × 3                                                                           2 × 10.sup.19                                                                   99     1280                                   13    1350 × 2                                                                           1800 × 3                                                                           2 × 10.sup.15                                                                   99     1420                                   14    1350 × 2                                                                           1800 × 3                                                                           7 × 10.sup.16                                                                   99     1300                                   15    1350 × 2                                                                           1800 × 3                                                                           2 × 10.sup.19                                                                   99     1420                                   16    1350 × 2                                                                           1700 × 3                                                                           9 × 10.sup.19                                                                   99     1250                                   17    1350 × 2                                                                           1800 × 3                                                                           2 × 10.sup.15                                                                   99     1310                                   18    1350 × 2                                                                           1800 × 3                                                                           7 × 10.sup.16                                                                   99     1410                                   19    1350 × 2                                                                           1800 × 3                                                                           2 × 10.sup.19                                                                   99     1270                                   20    1350 × 2                                                                           1800 × 3                                                                           9 × 10.sup.19                                                                   99     1250                                   21    1350 × 2                                                                           1600 × 1                                                                           9 × 10.sup.19                                                                   99     1040                                   22    1350 × 2                                                                           1800 × 3                                                                           2 × 10.sup.15                                                                   99     1410                                   23    1350 × 2                                                                           1800 × 3                                                                           7 × 10.sup.16                                                                   99     1300                                   24    1350 × 2                                                                           1800 × 3                                                                           3 × 10.sup.15                                                                   99     810                                    25    1300 × 4                                                                           1800 × 3                                                                           6 × 10.sup.15                                                                   99     870                                    26    1300 × 4                                                                           1800 × 3                                                                           2 × 10.sup.15                                                                   99     840                                    27    1300 × 4                                                                           1800 × 3                                                                           2 × 10.sup.15                                                                   99     820                                    28    1350 × 2                                                                           1500 × 3                                                                           9 × 10.sup.15                                                                   99     870                                    29    1350 × 2                                                                           1700 × 3                                                                           9 × 10.sup.15                                                                   99     840                                    ______________________________________                                    

As may be understood from the foregoing, by using a Si powder having an unpaired electron density of 10¹⁵ -10²⁰ /cm³, it is possible to control the unpaired electron density of the resulting Si₃ N₄ sintered body within the range of 10¹⁵ -10²¹ /cm³, and thus to obtain a close-grained high-strength sintered body of Si₃ N₄. In particular, by adding, as a nitrogen vacancy producing agent, an element having a valence of from +1 to +3 and a covalent bond radius RM such that its ratio to the covalent bond radius RSi of Si, say, (RM-RSi)/RSi is less than 0.5, and by sintering the mixture, it becomes possible to obtain a sintered body with even higher strength.

EXAMPLE 4

Commercial Si powder (A) was heat treated at 500° C. in air for 3 hours, and a Si powder having an unpaired electron density of 1×10¹⁶ /cm³ was obtained. Also, commercial Si powder (B) was heat treated at 500° C. in an argon atmosphere for 4 hours, and a Si powder having an unpaired electron density of 2×10¹⁹ /cm³ was obtained.

Added to each Si powder thus obtained were such sintering assistant and such nitrogen vacancy producing agent (both in powder form) as shown in Table 5 below. Mixing, molding and binder removal operations were carried out in the same way as in Example 1. It is noted that each nitrogen vacancy producing agent used is an element having a valence of from +1 to +3 and having a covalence bond radius such that its ratio to the covalent bond radius RSi of Si, that is, (RM-RSi)/RSi is less than 0.5, or a compound thereof.

                                      TABLE 5                                      __________________________________________________________________________                              Nitrogen vacancy                                                 Sintering assistant                                                                          producing agent                                       Sample                                                                              Si powder                                                                            Kind   Total (mol %)                                                                          Kind Loading (mol %)                                 __________________________________________________________________________     30   B     Y.sub.2 O.sub.3                                                                       0.5     Cu   1                                               31   B     Y.sub.2 O.sub.3 /MgO                                                                  5       Cr.sub.2 C                                                                          1                                               32   B     Pr.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                    10      TiN  1                                               33   B     Y.sub.2 O.sub.3                                                                       0.5     CuO  5                                               34   B     Y.sub.2 O.sub.3                                                                       0.5     TiO.sub.2                                                                           10                                              35   A     Y.sub.2 O.sub.3                                                                       2.5     CuO  2.5                                             36   A     Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                     7.5     Cr.sub.2 C                                                                          2.5                                             37   A     Y.sub.2 O.sub.3 /MgO                                                                  5       TiO.sub.2                                                                           5                                               38   A     Sm.sub.2 O.sub.3 /Nd.sub.2 O.sub.3                                                    10      Cr.sub.2 C                                                                          5                                               39   A     Yb.sub.2 O.sub.3                                                                      2.5     TiO.sub.2                                                                           7.5                                             40   A     Yb.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                    7.5     TiN  7.5                                             41   A     Y.sub.2 O.sub.3 /MgO                                                                  5       V.sub.2 C                                                                           10                                              42   A     Sm.sub.2 O.sub.3 /Nd.sub.2 O.sub.3                                                    10      Cr.sub.2 N                                                                          10                                              43   B     Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                     0.4     Cu   5                                               44   B     Sm.sub.2 O.sub.3 /Nd.sub.2 O.sub.3                                                    11      Cu   5                                               45   A     Y.sub.2 O.sub.3                                                                       0.05    Cu   5                                               46   A     Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                     16      Cu   5                                               47   B     Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                     5       Cu   0.4                                             48   A     Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                     5       Cu   16                                              __________________________________________________________________________

Next, each of the shaped bodies was subjected to nitriding treatment in a nitrogen stream at a temperature of 1300°-1350° C. for 2-4 hours. Then, sintering operation was carried out at a temperature of 1600°-1800° C. for 3 hours. Features of respective sintered bodies of Si₃ N₄, including unpaired electron density, relative density, and three-point bending strength, are shown in Table 6.

                  TABLE 6                                                          ______________________________________                                                                   Unpaired      3-point                                                Sinter    electron                                                                              Relative                                                                              bend                                         Nitriding conditions                                                                               density                                                                               density                                                                               strength                               Sample                                                                               (°C. × hr)                                                                  (°C. × hr)                                                                  (/cm.sup.3)                                                                           (%)    (MPa)                                  ______________________________________                                         30    1300 × 4                                                                           1800 × 3                                                                           7 × 10.sup.16                                                                   99     1120                                   31    1300 × 4                                                                           1600 × 3                                                                           2 × 10.sup.19                                                                   99     1050                                   32    1300 × 4                                                                           1800 × 3                                                                           9 × 10.sup.15                                                                   99     1210                                   33    1350 × 2                                                                           1700 × 3                                                                           2 × 10.sup.15                                                                   99     1100                                   34    1350 × 2                                                                           1600 × 3                                                                           7 × 10.sup.16                                                                   99     1100                                   35    1350 × 2                                                                           1800 × 3                                                                           2 × 10.sup.19                                                                   99     1250                                   36    1350 × 2                                                                           1800 × 3                                                                           9 × 10.sup.19                                                                   99     1200                                   37    1350 × 2                                                                           1700 × 3                                                                           2 × 10.sup.15                                                                   99     1100                                   38    1300 × 3                                                                           1800 × 3                                                                           7 × 10.sup.16                                                                   99     970                                    39    1350 × 2                                                                           1800 × 3                                                                           2 × 10.sup.15                                                                   99     1010                                   40    1350 × 2                                                                           1800 × 3                                                                           7 × 10.sup.16                                                                   99     890                                    41    1350 × 2                                                                           1600 × 3                                                                           2 × 10.sup.19                                                                   99     1160                                   42    1350 × 2                                                                           1800 × 3                                                                           9 × 10.sup.19                                                                   99     920                                    43    1350 × 2                                                                           1600 × 3                                                                           8 × 10.sup.15                                                                   99     900                                    44    1350 × 2                                                                           1800 × 3                                                                           2 × 10.sup.15                                                                   99     900                                    45    1350 × 4                                                                           1800 × 3                                                                           8 × 10.sup.16                                                                   99     820                                    46    1350 × 4                                                                           1700 × 3                                                                           9 × 10.sup.15                                                                   99     810                                    47    1350 × 4                                                                           1800 × 3                                                                           3 × 10.sup.16                                                                   99     815                                    48    1350 × 2                                                                           1800 × 3                                                                           3 × 10.sup.19                                                                   99     810                                    ______________________________________                                    

From the above results, it is found that a close-grained, higher strength sintered body of Si₃ N₄ can be obtained if the proportion of the sintering assistant(s), as a whole, is within a range of from 0.1 to 15 mol %, and if the proportion of the nitrogen vacancy producing agent is within a range of from 0.5 to 15 mol %.

EXAMPLE 5

Commercial Si powder (A) was heat treated at 500° C. in air for 3 hours, and a Si powder having an unpaired electron density of 1×10¹⁶ /cm³ was obtained. Also, commercial Si powder (B) was heat treated at 500° C. in an argon atmosphere for 4 hours, and a Si powder having an unpaired electron density of 2×10¹⁹ /cm³ was obtained.

Added to each Si powder thus obtained were such sintering assistant and such nitrogen vacancy producing agent as shown in Table 7 below. Mixing, molding and binder removal operations were carried out in the same way as in Example 1. It is noted that each nitrogen vacancy producing agent used is an element having a valence of from +1 to +3 and having a covalence bond radius such that its ratio to the covalent bond radius RSi of Si, that is, (RM-RSi)/RSi is less than 0.5, or a compound thereof.

                  TABLE 7                                                          ______________________________________                                                               Nitrogen vacancy                                                  Sintering assistant                                                                         producing agent                                                Si                  Total          Loading                               Sample                                                                               powder   Kind       (mol %)                                                                              Kind     (mol %)                               ______________________________________                                         45    A        Y          0.5   Cu (powder)                                                                             5                                                    (stearate)                                                      46    A        Y          0.5   Cr (powder)                                                                             5                                                    (alkoxide)                                                      47    A        Y/Al       0.5   Cu (powder)                                                                             5                                                    (laurate)                                                       48    A        Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                         5     Cu (stearate)                                                                           1                                                    (powder)                                                        49    B        Sm.sub.2 O.sub.3 /Nd.sub.2 O.sub.3                                                        5     Cu (alkoxide)                                                                           1                                                    (powder)                                                        50    B        Y.sub.2 O.sub.3                                                                           5     Cu (laurate)                                                                            1                                                    (powder)                                                        51    B        Y.sub.2 O.sub.3                                                                           0.5   Cu (alkoxide)                                                                           1                                                    (powder)                                                        52    B        Y.sub.2 O.sub.3                                                                           0.5   Cu (alkoxide)                                                                           1                                                    (powder)                                                        53    B        Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                         8     Cu (powder)                                                                             0.8                                                  (powder)                                                        54    B        Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                         8     Cu (powder)                                                                             0.8                                                  (powder)                                                        55    B        Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                         8     Cu (powder)                                                                             11                                                   (powder)                                                        ______________________________________                                    

Next, each of the shaped bodies was subjected to nitriding treatment in a nitrogen stream at 1350° C. for 2 hours. Then, sintering operation was carried out at a temperature of 1600°-1800° C. for 3 hours as shown in Table 8 below. Features of respective sintered bodies of Si₃ N₄ obtained, including unpaired electron density, relative density, and three-point bending strength, are shown in Table 8.

                  TABLE 8                                                          ______________________________________                                                     Unpaired                                                                   Sinter    electron  Relative                                                                               3-point bend                                       conditions                                                                               density   density strength                                   Sample  (°C. × hr)                                                                  (/cm.sup.3)                                                                              (%)     (MPa)                                      ______________________________________                                         45      1800 × 3                                                                           2 × 10.sup.19                                                                      99      1200                                       46      1800 × 3                                                                           9 × 10.sup.19                                                                      99      1210                                       47      1800 × 3                                                                           2 × 10.sup.15                                                                      99      1210                                       48      1800 × 3                                                                           2 × 10.sup.16                                                                      99      1190                                       49      1800 × 3                                                                           7 × 10.sup.16                                                                      99      1220                                       50      1800 × 3                                                                           2 × 10.sup.19                                                                      99      1150                                       51      1800 × 3                                                                           9 × 10.sup.19                                                                      99      1200                                       52      1700 × 3                                                                           9 × 10.sup.19                                                                      99      880                                        53      1600 × 3                                                                           8 × 10.sup.14                                                                      99      860                                        54      1800 × 3                                                                           8 × 10.sup.14                                                                      99      820                                        55      1800 × 3                                                                           2 × 10.sup.14                                                                      99      820                                        ______________________________________                                    

The above results tell that addition of sintering assistant and nitrogen vacancy producing agent in the form of stearate, laurate, or alkoxide is more advantageous than adding in the form of powder in that a smaller amount of such addition can result in production of a higher-strength Si₃ N₄ sintered body.

INDUSTRIAL APPLICABILITY

According to the present invention, a silicon powder, a material costing about 1/10 of Si₃ N₄ powder, rather than using the costly Si₃ N₄ powder is used. As a result, it is possible to provide a close-grained high strength sintered body of silicon nitride at a lower cost by reaction sintering which takes considerably shorter time than a conventional technique. 

What is claimed is:
 1. A sintered body of silicon nitride having an unpaired electron density of 10¹⁵ /cm³ to 10²¹ /cm³, a relative density of 99% or more and a bending strength of 800 MPa or more, produced by reaction sintering silicon powder, wherein the sintered body contains at least one of a rare earth element, Al, Mg and Ca, and a second element other than the rare earth element, Al, Mg and Ca, having a valence of from +1 to +3 and a covalent bond radius RM which is in such a relationship with the covalent bond radius RSi of silicon as (RM-RSi)/RSi<0.5.
 2. A sintered body of silicon nitride as defined in claim 1, wherein the second element having a valence of from +1 to +3 is at least one of elements Sc, Ni, V, Cr, Mn, Fe, Co, Cu, Ti, Zn, Ga, and Ge.
 3. A sintered body of silicon nitride as defined in claim 1, wherein the sintered body contains 0.1 to 15 mol % of at least one of rare earth element, Al, Mg, and Ca in terms of element value, and 0.5 to 15 mol % of the second element having a valence of from +1 to +3 in terms of element value.
 4. A sintered body of silicon nitride as defined in claim 1, wherein the sintered body contains 0.5 to 10 mol % of at least one of rare earth element, Al, Mg, and Ca in terms of element value, and 1 to 10 mol % of the second element having a valence of from +1 to +3 in terms of element value.
 5. A sintered body of silicon nitride as defined in claim 2, wherein the sintered body contains 0.1 to 15 mol % of at least one of rare earth element, Al, Mg, and Ca in terms of element value, and 0.5 to 15 mol % of the second element having a valence of from +1 to +3 in terms of element value.
 6. A sintered body of silicon nitride as defined in claim 2, wherein the sintered body contains 0.5 to 10 mol % of at least one of rare earth element, Al, Mg, and Ca in terms of element value, and 1 to 10 mol % of the second element having a valence of from +1 to +3 in terms of element value.
 7. A method of producing a sintered body of silicon nitride having a relative density of 99% or more and a bending strength of 800 MPa or more, which comprises annealing a commercially available silicon powder in a temperature range of from 300 to 800° C. in other than nitrogen atmosphere for 1 to 5 hours to obtain a silicon powder having an unpaired electron density of 10¹⁵ /cm³ to 10²⁰ /cm³, mixing said silicon powder with a compound of at least one of a rare earth element, Al, Mg and Ca as a sintering assistant and a second element other than the rare earth element, Al, Mg and Ca, having a valence of from +1 to +3 and a covalent bond radius RM which is in such a relationship with the covalent bond radius RSi of silicon as (RM-RSi)/RSi<0.5 as a nitrogen vacancy producing agent, and then subjecting the resultant mixture to reaction sintering.
 8. A method of producing a sintered body of silicon nitride as defined in claim 7, wherein the atmosphere is of air, hydrogen, or argon, or a vacuum of not more than 10 torr.
 9. A method of producing a sintered body of silicon nitride as defined in claim 7, wherein the nitrogen vacancy producing agent is at least one element selected from among Sc, Ni, V, Cr, Mn, Fe, Co, Cu, Ti, Zn, Ga, and Ge, or a compound thereof.
 10. A method of producing a sintered body of silicon nitride as defined in claim 7, wherein the sintering assistant and/or nitrogen vacancy producing agent is an oxide, nitride, carbide, alkoxide, stearate, or laurate of any such element as aforementioned.
 11. A method of producing a sintered body of silicon nitride as defined in claim 7, wherein the resultant mixture of silica powder with the sintering assistant and nitrogen vacancy producing agent is subjected to nitriding treatment while being kept at 1300°-1400° C. in a nitrogen atmosphere, the nitrided product being then sintered at 1500°-1800° C.
 12. A method of producing a sintered body of silicon nitride as defined in claim 8, wherein the resultant mixture of silica powder with the sintering assistant and nitrogen vacancy producing agent is subjected to nitriding treatment while being kept at 1300°-1400° C. in a nitrogen atmosphere, the nitrided product being then sintered at 1500°-1800° C.
 13. A method of producing a sintered body of silicon nitride as defined in claim 2, wherein the sintering assistant is mixed in an amount of 0.1 to 15 mol % in terms of element value and the nitrogen vacancy producing agent is mixed in an amount of 0.5 to 15 mol % in terms of element value.
 14. A method of producing a sintered body of silicon nitride as defined in claim 13, wherein the resultant Si powder with the sintering assistant and/or nitrogen vacancy producing agent is subjected to nitriding treatment while being kept at 1300°-1400° C. in a nitrogen atmosphere, the nitrided product being then sintered at 1500°-1800° C.
 15. A method of producing a sintered body of silicon nitride as defined in claim 8, wherein the sintering assistant is mixed in an amount of 0.1 to 15 mol % in terms of element value and the nitrogen vacancy producing agent is mixed in an amount of 0.5 to 15 mol % in terms of element value.
 16. A method of producing a sintered body of silicon nitride as defined in claim 8, wherein the nitrogen vacancy producing agent is at least one element selected from among Sc, Ni, V, Cr, Mn, Fe, Co, Cu, Ti, Zn, Ga, and Ge, or a compound thereof.
 17. A method of producing a sintered body of silicon nitride as defined in claim 13, wherein the nitrogen vacancy producing agent is at least one element selected from among Sc, Ni, V, Cr, Mn, Fe, Co, Cu, Ti, Zn, Ga, and Ge, or a compound thereof.
 18. A method of producing a sintered body of silicon nitride as defined in claim
 8. wherein the sintering assistant and/or nitrogen vacancy producing agent is an oxide, nitride, carbide, alkoxide, stearate, or laurate of any such element as aforementioned.
 19. A method of producing a sintered body of silicon nitride as defined in claim 13, wherein the sintering assistant and/or nitrogen vacancy producing agent is an oxide, nitride, carbide, alkoxide, stearate, or laurate of any such element as aforementioned. 